This invention relates to processes for forming glass fibers. More particularly, this invention relates to methods for forming ultrafine glass fibers using spinner disc techniques in combination with a relatively low input feed temperature of glass into the fiberizing disc and a temperature above the liquidus temperature of the glass outside of the fiberizing disc.
One of the more usual methods for producing glass fibers is by the so-called rotary or centrifugal process. In the rotary process, a molten stream of glass is deposited on a rapidly rotating rotor which has at its periphery a rim with a plurality of openings formed therein through which the molten glass issues in the form of filaments. As the filaments issue from the openings at the rotary periphery, they are subjected to the action of a high velocity, high temperature gaseous blast to attenuate the filaments into fine diameter fibers. A typical reference describing this process is U.S. Pat. No. 3,190,736. The spinner disc normally will have a solid bottom and a vertical, peripheral wall which contains a multiplicity of small orifices. The glass is always in a liquid state as it enters the fiberizer, and in this process it is critical that the glass passes through the orifices in the disc at a temperature well above the point of devitrification of the glass will occur. In the normal process, if the glass begins to crystalize, it will plug the orifices in the fiberizing disc. Therefore, the prior art rotary method for glass fiber protection requires that the input glass material to the spinner disc be in a molten state, a temperature well above the liquidus temperature of the glass material.
Another associated technology is the so-called flame attenuation technology. In this technique, the action of a high temperature gas stream upon extruded glass rods causes the heating and stretching of the rods such that glass fibers of a desired diameter are formed. Detailed explanations of this technology may be found in U.S. Pat. No. 2,607,075, assigned to Owens Corning, and U.S. Pat. Nos. 2,863,176 and 2,994,916, assigned to Johns Manville. Therein the extrusion of the primary filaments is limited to a single row. As such, this process is very inefficient from the standpoint of energy expenditure necessary to stretch out a single row of glass rods. Nevertheless, flame attenuated fiber has good qualities which are unequalled by fibers made from standard rotary techniques. However, the cold and brittle primary glass rod sometimes will prematurely break and cause the formation of "shot." This is, of course, a disadvantage to the flame attenuation process. The rotary technology, on the other hand, allows the production of a shot free fiber at energy consumption several times lower than required for flame attenuation. The standard rotary technology produces a relatively course glass fiber which is quite useful for insulating and acoustical purposes.